Update cython version from the docs
Update cython version from the docs

`cython==0.28.6` seems to be doing just fine on p4a until now.
https://github.com/kivy/python-for-android/blob/df60061/Dockerfile#L120

kivy/buildozer,kivy/buildozer,rnixx/buildozer,rnixx/buildozer,kivy/buildozer,rnixx/buildozer,kivy/buildozer,rnixx/buildozer